Method for determining an upward journey for all-wheel drive vehicles

ABSTRACT

The present invention relates to a method for controlling driving on a hill of an all-wheel drive vehicle, wherein vehicle acceleration is determined and the gravitational acceleration is measured. To improve the accuracy of the determination of the vehicle reference speed in all-wheel drive vehicles, the invention is characterized by the following steps: determining the acceleration at the secondary axle (Tc4wdHaAcc) from one or both of the two wheel speeds determining the deviation (Slope) between the acceleration at the secondary axle (Tc4wdHaAcc) and the measured acceleration (LoSenAcc) 
         filtering the determined deviation (SlopFilt) with a time constant (T1 slope ) comparing the deviation (Slope) with the filtered deviation (SlopFilt) determining driving situations representing the conditions ‘traction slip control is active’ or ‘traction slip control is not active’determining the slope in dependence on the comparison result and the driving situation.

The present invention relates to a method for controlling driving on ahill for all-wheel drive vehicles, wherein vehicle acceleration isdetermined and an acceleration of the vehicle is measured.

Modern motor vehicles are equipped with most various electronic systemsfor controlling and regulating the driving behavior. Examples of thesesystems are brake force control systems (ABS), driving stabilityprograms (ESP) and traction slip control systems (TCS, BTCS, or TCS).The control programs provided in the electronic unit of these systemsevaluate depending on the design a large number of sensors mounted intothe motor vehicle. The sensors are e.g. wheel speed sensors,longitudinal acceleration sensors, lateral acceleration sensors, and yawrate sensors. In traction slip control, the electronic unit ensures thatthe wheels accelerating the vehicle will not spin at great accelerationof the vehicle. On the one hand, this can be done by slowing down thespinning wheel, and/or by throttling the driving power (engine torquereduction).

DE-A 3809101 discloses a method and a circuit arrangement forcontrolling a TCS system with brake and engine management. For tractionslip control, the wheel brake is used and/or intervention into thecontrol of the driving engine is made with this method. To this end,traction slip control makes use of conventional components of thealready provided anti-lock system (ABS) among others. The communicationwith the required vehicle components (e.g. engine) can take place by wayof a vehicle data bus (CAN) known in the art. To implement the method,sensors for determining the wheel rotational behavior are evaluated bycircuits for signal processing purposes, and corrective signals forproducing electromagnetic hydraulic valves are generated and allowcontrolling the brake force. The electronics of the system requires theso-called vehicle reference speed V_(ref) to calculate the necessarycorrecting variable. This reference speed is supported by gradients andgenerally determined by the wheel speed of the slowest wheel. Thegradient is determined from the current engine torque and the emptyweight of the vehicle. The calculated gradient corresponds to thetheoretical vehicle acceleration at a high coefficient of friction inthe plane. When the all-wheel drive vehicles are equipped with anacceleration sensor (G-sensor), said sensor is additionally used fordefining the gradients. The maximum of calculated and measured gradientis determined to this end. When special driving situations areencountered, it may occur that one or more of the wheels will no longerconvey the current vehicle speed as one of the wheels is spinning. It isusual in this case that the spinning wheel is not taken intoconsideration for producing the vehicle reference speed V_(ref).

In addition, the problem is encountered in all-wheel drive vehicles thatall wheels can exhibit traction slip so that there is no indicator ofthe vehicle reference speed, which is usually determined from the wheelspeeds, in a case of driving, rendering it impossible to make adistinction e.g. between downhill driving and traction slip. Thisproblem cannot occur in a case of driving in vehicles where one axle isdriven because at least the non-driven axle can exhibit the absence oftraction slip. Thus, the wheels of the non-driven axle can always beused as an indicator of the reference speed.

DE 197 32 554 A1 discloses a method and a device for determining thespeed of an all-wheel driven vehicle. In this publication, individualwheel accelerations are related to the prevailing engine torque andcompared with one another in order to be able to initially detect thecondition that all wheels are spinning and then take remedies. It isdisadvantageous in this method that for different reasons the detectionthresholds must be chosen to be comparatively coarse to avoid errors inidentification, with the result that the detection is not very preciseand a road incline remains generally unnoticed.

When situations occur in which the wheel speeds alone are no longersufficient to reliably determine the vehicle reference speed, substitutestrategies are used. Substitute strategies permit solving individualproblems known in the art such as when the driver rides the vehicle at alow coefficient of friction with spinning wheels or when the vehicle isaccelerated under defined conditions when driving on a hill, especiallywhen driving downhill. The vehicle reference speed determined as asubstitute then becomes inaccurate at an increasing rate when conditionscoincide, and there is the need to solve also problems arising when anall-wheel driven vehicle suffers from an unstable wheel run duringdownhill driving, for example.

The influence of driving on a hill is only inappropriately or not at alltaken into consideration in the above-mentioned electronic systems forcontrolling the vehicle behavior. When driving down a slope, the actualvehicle acceleration is higher than the calculated vehicle acceleration[f(engine torque, vehicle mass)] and the measured vehicle acceleration(G-sensor or acceleration sensor) shows an acceleration value falsifiedby the gradient component. To compensate the slope, an invariablecomponent or a fixed gradient has been added so far to the calculatedgradient under defined conditions. This type of a slope correction isnecessary to rule out a too low reference speed during downhill driving.Said slope correction is, however, linked to conditions that may causeshortcomings on different road surfaces.

An object of the invention is to disclose a method of determining aslope and a method of adapting the vehicle reference speed to thedetermined slope, which achieve correct results in a reliable manner andwith low effort.

This object is achieved with the features of the independent claims.Dependent claims are directed to preferred embodiments of the invention.

Apart from determining the vehicle reference speed from the signals ofone or two wheel sensors on the secondary axle, the vehicleacceleration, especially the longitudinal vehicle acceleration, isdetermined indirectly or directly from the wheel signals and related ina comparison to an acceleration measured by means of a sensor. Thatvehicle axle is referred to as the secondary axle that can be connectedin an all-wheel drive vehicle. As this occurs, the effect is utilizedthat when driving on a hill the difference between the axle accelerationand G-sensor acceleration indicates the slope. The relationslope (Slope)=axle acceleration (from the wheel rotationalspeeds)−measured acceleration (G-sensor)applies.

As the wheel acceleration is directly related to the vehicleacceleration, the determined slope is correct only with a slip-free runof the wheels. Therefore, ‘stable’ wheels are defined, and the actualslope is determined in response to the respective situation.

The detection of stable wheels founds on the knowledge that the slopevariations of roadways change only at a relatively slow rate. Therefore,only the low-frequency component of the determined signal is used forthe slope (Slope). The low-frequency component is extracted by filteringthe slope with a time constant T1_(slope) in a low-pass filter of firstorder.Filtered slope (SlopeFilt)=PT1_Filt(Slope)

With the two signals ‘slope’ and ‘filtered slope’ the quality isestimated with respect to stable wheels and, thus, the determined slope.The quality estimation and the determination of the slope is executed independence on the respective driving situation, i.e. whether tractionslip control is active or not active, or the vehicle is or is notsubjected to a traction slip control operation.

A first allowable variance (SlopeVar) is produced by filtering theresult of comparison between slope and filtered slope as an indicator ofthe quality. The first variance is determined by low-pass filtering fromthe r.m.s. deviation of the current deviation (Slope) and the filtereddeviation (SlopeFilt). Further, a threshold valueTcs0_est_slope_var_limit of the variance is empirically determined andtaken into consideration in the learning or detection strategy for‘stable wheel rotational behavior’. When the determined value of thefirst variance is below the threshold value, stable wheel rotationalbehavior is assumed when traction slip control is not active. Thedetermined slope can be used for the determination of the slopeSlopeFilt or a saved slope SlopeSave and the related gradientadaptation.

When the system is under active traction slip control, meaning in a TCSengine control operation or BTCS brake control operation, externaldisturbances, such as quantities describing the road surface (lowcoefficient of friction, e.g. ice) can lead the testing of plausibilityto results mirroring stable conditions although unstable wheel runsprevail. Therefore, an unstable wheel rotational behavior is alwaysassumed when traction slip control is active, and a slope is allowedonly according to defined rules (linguistic terms). The rules are usedto avoid a slope, which is estimated wrongly at a low coefficient offriction, for example. To this end, a second variance (TorqueVar) isfavorably determined which represents the acceleration of the secondaryaxle in dependence on the engine torque. The second variance (TorqueVar)is evaluated in conjunction with the first variance (SlopeVar). In adriving situation where traction slip control is active, it is thenpossible to make a distinction between a quantity (downgrade force whendriving on a hill) that is produced externally and drives the vehicleand a quantity causing an unstable wheel run (excessive torque at a lowcoefficient of friction) in dependence on the result of the evaluation.To this end, a threshold value Tcs0_est_torque_var limit of the secondvariance (TorqueVar) is determined empirically and taken into account inthe learning or detection strategy for ‘stable wheel rotationalbehavior’. When the determined value of the second variance assumes adefined ratio to the value of the first variance, a stable wheelrotational behavior is assumed when the traction slip control is active.Testing or rendering plausible whether the acceleration at the secondaryaxle (Tc4wdHaAcc) indicates the actual vehicle acceleration is carriedout by determining whether the determined value of the second variance(TorqueVar) reaches or exceeds e.g. 1.5 to 2.5 times the value of thefirst variance (SlopeVar). When the conditions ‘second variance(TorqueVar)=1.5 to 2.5*first variance (SlopeVar)’ and ‘first varianceSlopeVar<Tcs0_est_slope_var_limit’ are satisfied, the filtered deviation(SlopeFilt) is trusted as being the true slope, and the saved slope(SlopeSave) is adjusted to it.

To preclude errors in the determination of the slope, learning of theslope is favorably terminated, with traction slip control active, whenthe first variance (SlopeVar) and the second variance (TorqueVar) reachor fall below at least one bottom threshold valueTcs0_est_slope_var_limit and/or Tcs0_est_torque_var_limit, and therespective threshold value of the variances can be equal or different.When the first variance (SlopeVar) and the second variance (TorqueVar)reach or fall below the threshold value Tcs0_est_slope_var_limit, thelearning or detection strategy is terminated in order to excite thesystem of the traction slip control unit. The engine torque is modulatedby way of the engine control unit for excitation.

The method advantageously provides that a saved slope (SlopeSave) isallowed in the presence of unstable wheel behavior when the unstablewheel behavior is determined or indicated by the relation ‘firstvariance (SlopeVar)≧Tcs0_est_slope_var_limit’.

In this arrangement, the saved slope (SlopeSave) is produced independence on the filtered deviation (SlopeFilt). The saved slope isthen weighted with a coefficient of confidence MyOfSlope which ispreferably determined in dependence on the engine torque requested bytraction slip control, the current engine torque variation, the firstvariance, the distance between the fastest and the slowest wheel, therate of deviation of the estimated slope (SlopeFilt) and the slope(SlopeFilt) limited by frequency.

When the slope has been safely determined, adaptation or modification ofthe vehicle reference speed to the driving situation takes place. Tothis end, a gradient derived from the determined slope is added to thereference gradient of the vehicle reference speed, i.e., raised by thedetermined slope.

One embodiment is illustrated in the accompanying drawing and will bedescribed in detail in the following.

In the drawing,

FIG. 1 is a diagrammatic view of a learning or detection strategyoutside of traction slip control.

FIG. 2 is a diagrammatic view of the learning or detection strategy whentraction slip control is active.

The slope offset degree_(slope) is composed of two components, theestimated slope C_(slope) and a safety constant C_(SAFE). When theacceleration sensor or G-sensor is defective, it is not possible toassess a slope.

The determination of C_(SLOPE) is illustrated after a short descriptionof the used base signals. Thereafter will follow information forcalculating the safety constant.

Base Signals

In the non-recursive approach chosen herein, the difference between theacceleration of the wheels of the secondary axle, e.g. the rear wheelsTc4wdHaAcc and the G-sensor acceleration LoSenACC according to therelationSlope=Tc4wdHaAcc−LoSenACCis taken as an assessed value for the slope, with Slope representing theestimated slope of a vehicle driving forward.

Tc4wdHaAcc must mirror the true vehicle acceleration in order that Slopecan be used as an assessed value for the slope, This applies only withstably running wheels of the secondary axle. When the wheels do not runstably, i.e. when the wheels are exposed to slip, the true slope isunknown.

Functional Description

The learning or detection strategy used for determining the slope forvehicles is carried out at least in dependence on two driving situationsrepresentative of the conditions ‘traction slip control is active’ or‘traction slip control is not active’. FIG. 1 shows the learning ordetection strategy outside of traction slip control. Said strategy usesa testing of plausibility of the torque rise Slope for detecting stablyrunning wheels. It is utilized that slope variations of roads can changeat a certain rate only within limits. To render Slope plausible, thesignal is filtered with a time constant of T1_(SLOPE)=k₁[msec] by meansof a PT1-filter of first order and compared to the slope variationSlope. If Slope differs greatly from SlopeFilt, then Slope will changemore than the incline or normal roads, and unstable wheels can beconcluded in the presence of major deviations. As an indicator of thedeviation, the variance of Slope is used which is the result of alow-pass filtering of the r.m.s. deviation of current and filtered slopewith T1_(VAR)=k₂[msec]. With SlopeVar≦the threshold valueTcs0_est_slope_var_limit, SlopeFilt can be used as an assessed value ofslope, that means that stable wheels prevail and that consequentlyTc4wdHaAcc mirrors the true vehicle acceleration. Thus, the signalSLOPE_STAB==1 is set. The threshold value Tcs0_est_slope_var_limit isfixed empirically. It is possible in this case to directly use theassessed value SlopeFilt or, alternatively, the value SlopeSave thatwill be described later as Cslope. If, however,SlopeVar>=Tcs0_est_slope_var_limit, wheels of unstable run can prevail.C_(Slope) will then be always set to the saved slope SlopeSave, and itsdetermination will be described later in a paragraph hereinbelow.

FIG. 2 shows the learning or detection strategy with an active tractionslip control. In contrast to the two distinguishing criteria ‘slopevariance is low’ or ‘slope variance is high’, with traction slip controlnot active, said strategy has four distinguishing criteria founding onthe inclusion of a second variance TorqueVar based on the engine torque.This second variance TorqueVar is an indicator of the excitation of thesystem, which must be sufficiently excited for the slope assessment tomake a distinction between the speed pattern of wheels on ice and thespeed pattern of wheels when driving downhill. TorqueVar is so designedthat SlopeVar and TorqueVar on ice show a similar variation, while theyshow a different variation on a high coefficient of friction.

As TorqueVar shall be identical with SlopeVar only on ice, the followingassumptions are allowable for the calculation of TorqueVar:

-   -   1. A constant propulsive force {overscore (F)}_(V) is generated.        In this case, the vehicle acceleration LoSenACC also is constant        at ACC_(const).    -   2. The driving track is rigid and unable to vibrate. The mass of        inertia J is known.

When the acceleration at the secondary axle in dependence on the enginetorque is expressed according to the relation${Tc4wdHaAcc} \sim {\frac{ActTorque}{J} - \frac{{\overset{\_}{F}}_{V}r_{eff}}{J}}$and defines the operating point of the engine control unit as a low-passfiltered signal of first order of the engine torque, the varianceTorqueVar results from the derivative of the relationsSlope = Tc4wdHaAcc − LoSenACC  with${Tc4wdHaAcc} \sim {\frac{ActTorque}{J} - \frac{{\overset{\_}{F}}_{V}r_{eff}}{J}}$so that${{Slope} = {\frac{ActTorque}{J} - \frac{{\overset{\_}{F}}_{V}r_{eff}}{J} - {LoSenACC}}},$wherein the constant components ACC_(const), {overscore(F)}_(V)r_(eff)/J are without influence on the variance. Under the aboveassumptions, “SlopeVar” can only be expressed in dependence on theengine torque because Tc4wdHaAcc is proportional to$\frac{ActTorque}{J}$applies.

A high TorqueVar and, simultaneously, low SlopeVar then indicates stablewheels (FIG. 2, quadrant 11). On the other hand, as FIG. 2 shows,unstable wheels are assumed in general with a high SlopeVar irrespectiveof TorqueVar (quadrants 12, 13). With a low TorqueVar and low SlopeVar,a decision on stable or unstable wheels is not possible (quadrant 14).The slope assessment must be disabled in this case until the system hasbeen excited to a sufficient degree again.

The system is considered as sufficiently excited when SlopeVar wouldsafely exceed the threshold Tcs0_est_slope_var_limit on ice due totorque modulation alone. To this end, the second variance TorqueVarbased on the engine torque must also be >Tcs0_est_slope_var limit. Dueto the modulation of the engine torque by way of the quantity‘TorqueExcit’, e.g. the wheels of the secondary axle will be afflictedby slip at low coefficients of friction. Acceleration values of thewheels will develop that are considerably above the vehicleacceleration, that means the wheel accelerations generally follow themodulation of the engine torque. If, however, the modulation of theengine torque takes place at a time of downhill driving at a highcoefficient of friction, the vehicle acceleration will basically followthe wheel accelerations, that means the wheel accelerations will notfollow the modulation of the engine torque. In this arrangement,TorqueExcit can be understood as minimal torque variation for asufficient system excitation.

When the first variance SlopeVar falls below the learning thresholdTcs0_est_slope_var_limit (transition from quadrant 11 to quadrant 14),then torque modulation will be compelled in that the system iscontrolled downwards to adopt a value below the minimum value of thequantity TorqueExcit. This causes active excitation of the system again.

Slope Offset C_(Slope)

In dependence on SlopeVar, the calculation rule for C_(Slope) results aslisted in the following table: TABLE 1 Slope Offset C_(Slope)TCS-control is not TCS-control is active active SlopeVar < Tcs0_(—)C_(Slope) = SlopeFilt or C_(Slope) = SlopeSave est_slope_var_limitSlopeSave SlopeVar >= Tcs0_(—) C_(Slope) = SlopeSave est_slope_var limit

SlopeSave is determined from the signals of the last slope learnt withstable wheels, consequently, from SlopeFilt and by way of a coefficientof confidence MyOfSlope representative of a quantity indicating to whatextent SlopeSave is adjusted to the filtered slope variation SlopeFilt.More specifically, MyOfSlope indicates a factor showing to what extentthere is confidence in the determined signal SlopeFilt representing theactual slope value. The stronger the confidence in the value SlopeFiltis, the more SlopeSave is adjusted to this filtered slope value.

The weighting of MyOfSlope must be chosen to be so that:

-   -   a too high assessed slope at low coefficients of friction is        avoided,    -   a slope correction when driving downhill with spinning wheels on        a downgrade is still possible,    -   permanent controls during downhill driving can be terminated by        adjustment of the slope.

The coefficient of confidence MyOfSlope is determined for two cases tobe distinguished. These cases are:

-   -   a. TCS engine control and    -   b. outside of TCS engine control.

The coefficient of confidence MyOfslope is defined based on the abovedescribed learning conditions (functional description), i.e. outside ofthe TCS engine control,

-   -   c. the first variance SlopeVar must be low        (Tc4wdHaAcc≈AcCwheels)        that means during TCS engine control    -   d. the first variance must be low (Tc4wdHaAcc≈AcCwheels)    -   e. the second variance TorqueVar must be high        (TorqueVar≈SlopeVar (at a low coefficient of friction) or        TorqueVar>SlopeVar (during downhill driving)    -   f. and further linguistic terms must be satisfied, such as the        distance between the fastest wheel and the slowest wheel must        comply to defined criteria in terms of the speed, the engine        torque requested by traction slip control, the current engine        torque variation, the first variance SlopeVar, the rate of the        deviation of the assessed slope (Slope) and the slope        (SlopeFilt) limited by frequency must satisfy defined criteria,        etc.

MyOfSlope is calculated from the Fuzzy quantities in dependence on theabove-mentioned criteria.

1-18. (canceled)
 19. Method for controlling driving on a hill forall-wheel drive vehicles, wherein vehicle acceleration is determined andan acceleration of the vehicle is measured, wherein the steps ofdetermining the acceleration at the secondary axle (Tc4wdHaAcc) from oneor both of the two wheel speeds determining the deviation (Slope)between the acceleration at the secondary axle (Tc4wdHaAcc) and themeasured acceleration (LoSenAcc) filtering the determined deviation(SlopFilt) with a time constant (T1_(Slope)) comparing the deviation(Slope) with the filtered deviation (SlopFilt) determining drivingsituations representing the conditions ‘traction slip control is active’or ‘traction slip control is not active’ determining the slope independence on the comparison result and the driving situation. 20.Method as claimed in claim 19, wherein the slope is determined by meansof at least one first allowable variance (SlopeVar) by filtering theresult of comparison.
 21. Method as claimed in claim 20, wherein thevariance (SlopeVar) is determined by low-pass filtering from the r.m.s.deviation of the current deviation (Slope) and the filtered deviation(SlopeFilt).
 22. Method as claimed in claim 19, wherein a thresholdvalue ‘Tcs0_est_slope_var_limit’ of the first variance (SlopeVar) isempirically determined.
 23. Method as claimed in claim 19, wherein thevalue of the filtered deviation (SlopeFilt) or of a saved slope(SlopeVar) is allowed as a slope in a driving situation where tractionslip control is not active and wherein the first variance (SlopeVar)≦thethreshold value (Tcs0_est_slope_var-limit.
 24. Method as claimed inclaim 19, wherein a second variance (TorqueVar) is determined whichrepresents the acceleration of the secondary axle in dependence on theengine torque.
 25. Method as claimed in claim 19, wherein the secondvariance (TorqueVar) and the first variance (SlopeVar) are evaluated.26. Method as claimed in claim 19, wherein in a driving situation wheretraction slip control is active, a quantity that is caused externally,drives the vehicle or represents an unstable wheel run (downhilldriving, riding on ice, etc.) is determined in dependence on theevaluation of the first and second variance.
 27. Method as claimed inclaim 19, wherein a threshold value Tcs0_est_torque_var_limit of thesecond variance (TorqueVar) is determined empirically.
 28. Method asclaimed in claim 19, wherein for testing whether the acceleration at thesecondary axle (Tc4wdHaAcc) indicates the actual vehicle acceleration,it is determined whether the determined value of the second variance(TorqueVar) reaches or exceeds at least 1.5 to 2.5 times the value ofthe first variance (SlopeVar).
 29. Method as claimed in claim 28,wherein when the condition ‘second variance (TorqueVar)=at least 1.5 to2.5*first variance (SlopeVar)’ is satisfied, the value of the filteredslope (SlopeFilt) is allowed as slope.
 30. Method as claimed in claim19, wherein the determination of the slope is terminated, with thetraction slip control active, when the first variance (SlopeVar) and thesecond variance (TorqueVar) reach or fall below at least one bottomthreshold value Tcs0_est_slope_var_limit’ orTcs_(—)0_est_slope_var_limit, respectively, and the respective thresholdvalue of the variances can be equal or different.
 31. Method as claimedin claim 29, wherein that the engine torque is modulated for excitingthe traction slip control unit in the case that the first variance(SlopeVar) reaches or falls below the threshold valueTcs0_est_slope_var_limit and the second variance (TorqueVar) reaches orfalls below the threshold value Tcs0_est_torque_var_limit.
 32. Method asclaimed in claim 19, wherein a saved slope (SlopeSave) is allowed whenan unstable wheel run is determined or indicated by the relation ‘firstvariance (SlopeVar)≧Tcs0_est_slope_var_limit’.
 33. Method as claimed inclaim 19, wherein the saved slope (SlopeSave) is determined independence on the filtered deviation (SlopeFilt) and a factor MyOfSlope.34. Method as claimed in claim 33, wherein the saved slope is weightedwith a coefficient of confidence MyOfSlope which is preferablydetermined in dependence on the engine torque requested by traction slipcontrol, the current engine torque variation, the first variance, thedistance between the fastest and the slowest wheel, the rate ofdeviation of the estimated slope (SlopeFilt) and the slope (SlopeFilt)limited by frequency.
 35. Method as claimed in claim 19, wherein thevehicle reference speed is adapted to a driving situation.
 36. Method asclaimed in claim 35, wherein the reference gradient of the vehiclereference speed is raised by addition of the determined slope.